Washers

What is a washer?

A washer is a thin plate (typically disk-shaped) with a hole (typically in the middle) that is normally used to distribute the load of a threaded fastener. Other uses are as a spacer, spring, wear pad, preload indicating device, and locking device. Washers usually have an outer diameter (OD) about twice the length of their inner diameter (ID).

High quality capscrews require hardened steel washers to prevent the loss of pre-load due to Brinelling after the torque is applied. While they may look similar, washers and gaskets are usually designed for different functions and made differently.
Washers are also important for preventing galvanic corrosion, particularly by insulating steel screws from aluminum surfaces.

Descriptions of Washer Types

Countersunk Finishing Washers

Countersunk finishing washers or "cup washers" are designed to provide a finished look with use of countersunk screws (usually flat or oval head). In Steel, they are available in Zinc plated, Black Zinc plated and Black Oxide plated. They are also available in 18-8 Stainless Steel.

Tooth Washers

A locking washer with teeth or prongs which extend from either the inside edge (Internal Tooth Washers), the outside edge (External Tooth Washers) or both (Internal/External Combination). The teeth maintain essential tension and oppose any loosening influences on the fastener. The flexed teeth absorb shock, vibration and slipping. In Steel, they are available in Zinc plated, Yellow Zinc plated, Black Zinc plated and Black Oxide. They are also available in 18-8 Stainless steel, 18-8 Stainless Steel Black Oxide plated and 410 Stainless steel.

Shoulder Washers

A flat washer with a shoulder around the center hole on one side. It is used to insulate a screw, wire or rivet from a panel. It can be used as a guide or a spacer. It is made of Type 6/6 Nylon which allows it to be a good electrical insulator with high strength.

Lock Washers

A helical spring lock washer is a flat washer with a cut on one side, slightly twisted so that the free height is approximately twice the thickness of the washer section. They have the function of providing greater bolt tension, providing hardened bearing surfaces to create more uniform torque control and providing protection against looseness resulting from vibration and corrosion. Lock washers are available in a variety of materials including steel, high alloy steel, 18-8 and 316 stainless steel. Types of plating include Zinc, Yellow Zinc, Black Oxide or have no plating at all. They are available in Regular, Heavy or High Collar styles. Lock washers that are fully certified to military standards are also available.

Flat Washers

Flat washers are used under the head of a bolt or screw or over a nut to provide a greater bearing surface. When tightening this will distribute the forces and prevent the bolt, screw or nut from breaking through the material. Flat washers are available in a variety of materials including mild steel, hardened alloy steel, Nylon 6/6 and 18-8 and 316 stainless steel. Types of plating include Zinc, Yellow Zinc, Black Zinc, Black Oxide, Galvanized or have no plating at all. There are a variety of styles of flat washers which differ mostly in the outside diameter dimension. The most common flat washers are the USS and SAE style, and are also available fully certified to military standards.